1. Field
The present invention relates to an optical device performing phase modulation and to an optical modulation method.
2. Description of the Related Art
In recent years, demand for introduction of next-generation 40-Gbps optical transmission systems is developing with the increasing amount of transmission traffic. It is necessary for the 40-Gbps optical transmission systems to realize transmission distances and spectral efficiencies equivalent to those of the 10-Gbps optical transmission systems in related art. In order to enable the realization, research and development of modulation methods having excellent Optical Signal-to-Noise Ratio (OSNR) performance and nonlinearity tolerance are promoted. Such modulation methods include Return-to-Zero Differential Phase Shift Keying (RZ-DPSK) and Carrier-Suppressed Return-to-Zero Differential Phase Shift Keying (CSRZ-DPSK).
Return-to-Zero Differential Quadrature Phase Shift Keying (RZ-DQPSK) has a relatively narrower spectrum (higher-spectral efficiency), so the RZ-DQPSK is expected to be a major candidate for a modulation method for the next-generation optical transmission system.
In the RZ-DQPSK, a DQPSK phase modulator includes two Mach-Zehnder interferometers composing I and Q arms and performs quadrature phase modulation. The I and Q arms each perform binary phase modulation on the basis of a data signal input. The phase modulator multiplexes the data signals modulated by the I and Q arms and supplies the multiplexed signal to a RZ modulator as a DQPSK modulation signal.
The RZ modulator performs RZ modulation on the DQPSK modulation signal supplied from the phase modulator on the basis of a clock signal input. The RZ modulator externally outputs the signal that is subjected to the RZ modulation as a RZ-DQPSK signal.